CD4+ T cells can be segregated into distinct subsets based on their pattern of cytokine production. CD4+ T cells that secrete IFN-g are termed Th1 cells while cells that secrete IL-4, IL-5 and IL-13 are termed Th2 cells. While Th1 cells are essential in mediating protection against a variety of intracellular infections they also are mediate pro-inflammatory responses associated with a variety of autoimmune diseases. Th2 cells have a role in counter-regulating Th1 responses and are associated with allergic and asthmatic disease. These studies examine the factors, which regulate how T helper responses are sustained in vivo.